The liquid crystal light valve (LCLV) is an optical-to-optical image transducer that is capable of accepting a low-intensity visible light image and converting it, in real time, into an output image with light from another source. Such devices have been used for optical data processing applications as well as large-screen projector displays.
The LCLV generally consists of a photoconductor, or photosensor, film or layer, and a liquid crystal layer. The two layers are separated by a light-blocking layer and a dielectric mirror. Adjacent the outer surfaces of the liquid crystal and photosensor layers are two transparent electrodes connected to a voltage source. The voltage source and electrodes serve to maintain a bias voltage across the photosensor and liquid crystal layers. Finally, a glass substrate is generally provided over the transparent electrode which is adjacent the liquid crystal layer.
The photosensor film, generally cadmium sulfide, serves as an imaging, light-controlled, voltage-modulator for the liquid crystal layer. In response to the input light pattern, the photosensor impedance lowers, thereby switching the bias voltage to the liquid crystal layer. This causes the liquid crystals to realign in a pattern corresponding to the light input image intensity by locally driving the liquid crystal layer above its electro-optic threshold. For real-time response, a liquid crystal layer between 2 and 6 .mu.m thick is generally used.
The LCLV is often employed with liquid crystal molecules having a twist of 45.degree. rather than the conventional 90.degree. of typical twisted-nematic liquid crystal displays. This is because the 90.degree. twisted-nematic does not modulate the intensity of the light beam effectively when both the polarizer and the crossed analyzer are on the same side of the liquid crystal, as is generally required in the reflection-mode light valve operation. A twist angle of 45.degree. gives the maximum modulation effect.
The LCLV having a photosensor layer comprising cadmium sulfide has a characteristic spectral input response having a peak photoresponse at 515 nm in the green. Thus, such a light valve is well suited to accept input from certain CRT phosphors as well as a 514 nm argon-ion laser line.
When addressed with a laser, very high resolution is possible because of the well-defined shape of the laser beam as compared to the spot shapes from a CRT. Line widths of 0.5 mils projected from the LCLV have been achieved, corresponding to a 2,000 TV-line display.
The present invention adds a new dimension to prior art LCLV technology by achieving for the first time the storage of light input images after the input is terminated.